Can bodies are typically formed from a sheet of metal which is rolled into a tube shape. The join in the tube is closed by a weld, for example. The side seam formed by the weld is often referred to as a side stripe. Can “ends” are fixed to a first end of the tube, the resultant can body is then filled and closed at the opposite end to produce a so-called three-piece can. Traditionally, three-piece cans are mainly used for the packaging of food products and range in diameter from 52 mm to 153 mm and over a height range of 38 mm to 178 mm.
Alternatively, a can body is formed by a drawing technique in which a punch forces metal through a die or series of dies to form a can body with an integral base or “bottom”. Sometimes, these can bodies are stretched further to increase the side wall by a technique known as “wall-ironing”. The drawn or drawn and wall-ironed can body is then filled and closed with a single end to produce a so-called two-piece can. Although two-piece cans are also used for the packaging of food products, largest numbers of two-piece cans are used for still and carbonated beverages. Two-piece beverage cans range in diameter from 52 mm to 84 mm and range in height from 88 mm to 204 mm.
It is well known to coat an internal side strip on a welded sheet metal pipe. U.S. Pat. No. 6,146,695 (FREY) is one example of this in which a thermoplastic material is melted and applied by a nozzle in metered quantities over the longitudinal weld. This patent also notes that clean sterilisation-resistant internal covering of the weld is of great importance in tinned metal cans used for the foodstuff industry. Any coating applied to such “food cans” also needs to be resistant to metal forming processes as well as any processing of the filled can. In this Frey patent, thermoplastic “hotmelt” material such as polyester is recommended as for its strength and foodstuff tolerance. The protective film in this patent is applied by a nozzle directly on the weld seam only. The tinned metal used at the time of filing this patent would have been 0.16 mm to 0.2 mm.
A lacquer spray machine for applying and evenly distributing a lacquer coating over the entire internal surface of a can body for a two piece can, i.e. having a side wall and integral base, was known since 2007 from CarnaudMetalbox “3200” spray machine, for example. The 3200 machine at that time applied interior lacquer to two piece can bodies ranging in size from 15 cl “202” to 50 cl “211”. 202 is 52 mm diameter, and the height of a 15 cl 202 can body is 88 mm. Similarly, 211 is 66 mm diameter, and the height of a 50 cl 211 can body is 168 mm.
In the 3200 machine, cans (i.e. can bodies) are received by an infeed assembly. Cans released by the infeed are positioned in “pockets” supplied by vacuum to base pads on a rotary turret. On the turret, cans are received by a turret can carrier and drawn onto the vacuum base pad (referred to below as “vacuum chuck”).
The 3200 machine has a spray station in which lacquer is applied to the inside of can bodies. In this 3200 machine, cans are indexed into the spray station for application of spray for a selected period of time. This spray machine has a pulley and belt drive. A can base pad pulley makes contact with a spinner belt that rotates the can at high speed, typically 2000 rpm to 2400 rpm. At 2000 rpm, there would be 3 can wraps, at 2400 rpm, 4 wraps would be required. A “can wrap” is a term for one can revolution whilst coating is being applied. During the spinning of the cans, a spray gun is activated and coating is applied by spraying and evenly distributing lacquer over the interior of the can.
Spray time is limited by gun reaction time; weight of coating media which the gun is capable of applying in the time period; acceleration of the vacuum base pad to desired speed through contact with its associated spinner belt; machine sensor reaction time; confirming the turret is in a spray position; and checking that the chuck is at the correct speed.
A conventional machine such as the 3200 is shown in FIGS. 1 to 3. Cans (i.e. can bodies) 2 pass along infeed tracking 4 and are fed in turn to pockets 6 of a main turret 10. Production speed is related to spray time and coating weight. Turret 10 is connected to and driven by a main turret index box 12 (FIG. 2). The turret motion is index-dwell-index etc. with spray coating occurring in the dwell period. At 350 cans per minute (“cpm”), an index box cam gives time per can of 0.171 seconds (60/350).
The index box cam revolves one revolution per can. The 3200 cam periods are 150° degrees for index movement and 210° degrees for dwell. Index movement time for this machine is therefore 150/360×60/350=0.071 seconds. Dwell time is 210/350×60/360=0.100 seconds.
Vacuum chucks 16 are mounted on the main turret 10 and arranged on a pitch circle diameter or “PCD” about the main turret centre 14. Each vacuum chuck 16 is equipped with a chuck spinner pulley 18 which receives drive through contact with a chuck spinner drive belt 20. The chuck spinner pulleys 18 move into and out of contact with the chuck spinner drive belt 20 during one full rotation of the main turret. For a 12 pocket machine this is 2.06 seconds (60/350×12) and for a 6 pocket machine this is 1.03 seconds. The main turret index box 12 and chuck spinner drive motor 22 plus idler pulley 23 are static mounted to the main frame 24, i.e. without allowing relative movement.
Finally it is noted that after application of the internal coating, cans are passed to a discharge turret 36 and then along discharge trackwork 38.
This 3200 machine is intended to operate at up to 350 cpm for a 12 pocket machine or 420 cpm for a 6 pocket machine. A problem with this machine is that the belt is often contaminated by coating due to over-spray. Further problems include loss of friction between spinner pulley 18 and belt 20, and loose (i.e. relaxed) control of wraps.
Spray machine station 30 comprises chuck spinner drive belt 20, chuck spinner motor 22 and spray gun 32 (which is mounted on support frame 34). Section X-X of FIG. 2 and enlarged view (FIG. 3) of the chuck spinner belt 20 show the spray machine station 30 and process area in more detail. Also visible in this section are main turret index box 12 and its associated index box drive motor 11. Hub 25 provides static mounting for a vacuum manifold (26) which supplies timed vacuum service to the vacuum chucks.
In this conventional machine, debris due to wear of the chuck spinner drive belt 20 contaminates the can 2 and spray process area. Clearly cans which are contaminated in the spray machine are rejected from the process. In addition, friction drive between the chuck spinner drive belt and chuck spinner pulley is unreliable such that skidding occurs. As a result, the number of can process revolutions during spraying has a degree of uncertainty, which limits machine production speed to 350 cpm for a 12 pocket machine or, in situations where can quality specification is less rigid, to 420 cpm for a 6 pocket machine as noted above.
The spray machine of the present invention seeks to eliminate contamination of the spray process, while enabling high speed production of cans (i.e. can bodies) with fully coated interiors and achieving tight can coating specifications. These can coating specification parameters include coating weight, distribution and metal/aluminium exposure.